Lithography systems are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical lithography system includes an illumination system, a reticle stage assembly that positions a reticle, an optical assembly and a wafer stage assembly that positions a semiconductor wafer. The illumination system includes an illumination source that generates an illumination beam, and an illumination optical assembly that directs the illumination beam at the reticle.
The size of the features within the images transferred from the reticle onto the wafer is extremely small. In order to increase the resolution of the features and decrease the size of the features within the images, there is a need to use an illumination source that generates smaller or shorter wavelengths of light. For example, extreme ultraviolet (EUV) radiation, including wavelengths in the 11 to 14 nm range, is being evaluated for use in lithography systems. For extreme ultraviolet lithography systems, the optical assembly typically includes one or more reflective, optical elements, e.g. mirrors.
With EUV lithography systems, an EUV source generates the EUV beam, while an illumination optical assembly directs the EUV beam at the reticle. A typical illumination optical assembly includes one or more mirror assemblies that reflect and direct the EUV beam at the reticle. Unfortunately, existing coatings for the mirror assembly only reflect a portion of the EUV beam. As a result thereof, the mirror assembly absorbs a portion of the EUV beam. This heats the mirror assembly and can deform the mirror assembly. Further, the deformation of the mirror assembly can adversely influence the EUV beam reflected off of the mirror assembly.